Just in case you still thought that biofuels such as ethanol, biodiesel and biogas might be a sustainable solution to stem this pesky climate change thing, some sobering thoughts from Hartmut Michel, an eminent molecular biologist from the University of Frankfurt am Main.

They are already being blamed for the widespread destruction of tropical forests (to grow the raw materials such as maize or beet) and the United Nations has warned that the global rush for biofuels could bring food shortages and increase poverty.

Concerns like these might be enough to clip the wings of the biofuel excitement but, by calculating how much useful energy you can theoretically extract from a field of crops destined to become fuels, Prof Michel has found another, more technical, problem. Simply put, there is not enough space to grow the crops we would need to replace our dependency on fossil fuels.

He started with the fact that photosynthesis, the process by which plants trap the energy pouring in from the Sun, is very inefficient. Only 1% of the energy that is absorbed turns into part of the plant. (It's worth pointing out that Prof Michel is something of an expert here - he was awarded the Nobel prize for chemistry in 1988 for working out the precise mechanics of photosynthesis.)

Further losses during the conversion process, often fermentation or simply burning, mean that only 0.4% of the energy of the sunlight that falls onto an area of land becomes useful biofuel energy. In addition, Prof Michel estimates that 50% of the energy contained in biogas or biodiesel has to come from conventional fossil fuels to make the fuel in the first place, so it is not carbon neutral.

By his calculation, in order to reproduce Germany's electricity consumption by burning biofuels, you would need an area of crops bigger than the whole country.

Such pessimism is not deterring governments from jumping onto the biofuel bandwagon: last year more than a third of the entire US maize crop went to ethanol for fuel, a 48% increase on 2005, and Brazil and China grew the crops on nearly 50m acres of land; the European Union has set a target to get 5.75% of fossil fuel used in traffic replaced with biofuels by 2010.

In his lecture today at the annual meeting of Nobel laureates in Lindau, Germany, he called on the EU to abandon its biofuel target and focus instead on other renewable energies, notably photovoltaic solar panels. He argued that these are more efficient by a factor of 50 to 100 with respect to energy conversion and would therefore require less land to harvest the same amount of energy as biofuels.

Perhaps farms of solar panels could be arranged in deserts across the world, he said, connected by superconducting wires to create a global network of clean electricity. A bit outlandish, he admitted, but much better than relying on the biofuel hype.